Van der Waals Antiferromagnets Co_1/3NbS₂ and Co_1/3TaS₂: Topological Magneto-Optics & Tunable Chiral/Nematic Phases

Co_1/3NbS₂ and Co_1/3TaS₂ are van der Waals antiferromagnets (AFMs) hosting layers of spins on 2D triangular lattices. They exhibit giant spontaneous Hall conductivity despite vanishing net magnetization, due to non-coplanar "tetrahedral" triple-Q AFM order with scalar spin chirality. Unlike collinear AFM order, this (and certain other) complex AFM spin configurations can allow for off-diagonal Hall conductivity, σ_xy, which in turn generates anomalous and topological Hall effects in transport studies. Crucially, σ_xy(ω) is frequency-dependent, and at optical frequencies it generates Kerr rotation and magnetic circular dichroism (MCD).  Thus, the full power of optics, including imaging and spectroscopy, can be applied to study complex AFM orders in Co_1/3NbS₂ and Co_1/3TaS₂.  Here we show [1], using light spanning infrared-ultraviolet (1-3 eV), that MCD is a powerful and incisive probe of chiral triple-Q AFM order. Measurements at different photon energies are compared with DFT calculations. Scanning MCD microscopy is used to directly image (and also write) chiral 3Q domains. In Co_1/3TaS₂, linear dichroism studies also reveal three-state (Z₃) nematicity arising from a single-Q stripe phase, as well as phases where chirality and nematicity coexist. A theory based on a continuous multi-Q manifold captures the emergence of these distinct magnetic phases, resulting from interplay between four-spin interactions and weak anisotropy.  [1] arXiv:2507.18829 (in press, PRL);  arXiv:2507.08148 (submitted)